Display, method for driving display, and electronic apparatus

ABSTRACT

Disclosed herein is a display including: a pixel array part configured to include pixels that are arranged in a matrix, each having an electro-optical element, a write transistor for writing a video signal, a drive transistor for driving the electro-optical element based on the video signal written by the write transistor, and a holding capacitor connected between the gate and source of the drive transistor, wherein the holding capacitor includes a first electrode, a second electrode disposed to face one surface of the first electrode for forming a first capacitor, and a third electrode disposed to face the other surface of the first electrode for forming a second capacitor, and the first capacitor and the second capacitor are connected in parallel to each other electrically.

CROSS REFERENCES TO RELATED APPLICATIONS

The present invention contains subject matter related to Japanese PatentApplication JP 2007-101282 filed in the Japan Patent Office on Apr. 9,2007, the entire contents of which being incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a display, a method for driving adisplay, and electronic apparatus, and particularly to a flat-type(flat-panel) display in which pixels, each including an electro-opticalelement, are arranged on rows and columns (in a matrix), a method fordriving the display, and electronic apparatus including the display.

2. Description of the Related Art

In recent years, in the field of displays for image displaying,flat-type display obtained by arranging pixels (pixel circuits) eachincluding a light-emitting element in a matrix is prevailing rapidly. Asthe flat-type display, an organic electroluminescence (EL) display isbeing developed and commercialized. The organic EL display includesorganic EL elements as the light-emitting elements of the respectivepixels. The organic EL element is a so-called current-drivenelectro-optical element whose light-emission luminance varies dependingon the value of the current flowing through the element, and is based ona phenomenon that light emission occurs in response to electric fieldapplication to an organic thin film.

The organic EL display has the following features. Specifically, theorganic EL display has low power consumption because the organic ELelement can be driven by an application voltage lower than 10 V.Furthermore, because the organic EL element is a self-luminous element,the organic EL display provides higher image visibility compared with aliquid crystal display, which displays an image by controlling, for eachpixel including a liquid crystal cell, the intensity of light from alight source (backlight) by the liquid crystal cell. In addition, theorganic EL display does not need to have an illuminating unit such as abacklight, which is necessary for the liquid crystal display, andtherefore reduction in the weight and thickness of the organic ELdisplay can be easily achieved. Moreover, the response speed of theorganic EL element is as high as several microseconds, which causes noimage lag in displaying of a moving image by the organic EL display.

As the drive system for the organic EL display, asimple-(passive-)matrix system or an active-matrix system can beemployed, similar to the liquid crystal display. However, a display ofthe simple-matrix system involves a problem that it is difficult torealize a large-size and high-definition display since thelight-emitting period of the light-emitting element is decreased due tothe increase of the scanning line (i.e. number of pixels) although theconfiguration thereof is simple.

For this reason, in recent years, a display of the active-matrix systemis being actively developed in which the current flowing through anelectro-optical element is controlled by an active element, such as aninsulated gate field effect transistor (typically, thin film transistor(TFT)), provided in the same pixel circuit as that including thiselectro-optical element. In the display of the active-matrix system, theelectro-optical element continues light emission over the one-frameperiod. This easily realizes a display having a large size and highdefinition.

It is generally known that the I-V characteristic (current-voltagecharacteristic) of an organic EL element deteriorates as the timeelapses (so-called age deterioration). In a pixel circuit including anN-channel TFT as a transistor for driving an organic EL element bycurrent (hereinafter, referred to as a “drive transistor”), the organicEL element is connected to the source side of the drive transistor.Therefore, the age deterioration of the I-V characteristic of theorganic EL element leads to change in the gate-source voltage Vgs of thedrive transistor, which results in change in the light-emissionluminance of the organic EL element.

A more specific description will be made below about this point. Thesource potential of the drive transistor is dependent on the operatingpoint of the drive transistor and the organic EL element. Thedeterioration of the I-V characteristic of the organic EL element variesthe operating point of the drive transistor and the organic EL element.Therefore, even when the same voltage is applied to the gate of thedrive transistor, the source potential of the drive transistor varies.This varies the source-gate voltage Vgs of the drive transistor, whichchanges the value of the current flowing through the drive transistor.As a result, the value of the current flowing through the organic ELelement also changes, which varies the light-emission luminance of theorganic EL element.

Furthermore, a pixel circuit employing a poly-silicon TFT involves, inaddition to the age deterioration of the I-V characteristic of theorganic EL element, changes over time in the threshold voltage Vth ofthe drive transistor and the mobility μ in the semiconductor thin filmserving as the channel of the drive transistor (hereinafter, referred toas “the mobility of the drive transistor”), and differences in thethreshold voltage Vth and the mobility μ from pixel to pixel due tovariation in the manufacturing process (variation in transistorcharacteristics among the respective drive transistors).

If the threshold voltage Vth and mobility μ of the drive transistor aredifferent from pixel to pixel, variation in the value of the currentflowing through the drive transistor arises on a pixel-by-pixel basis.Therefore, even when the same voltage is applied to the gate of thedrive transistor among the pixels, variation in the light-emissionluminance of the organic EL element among the pixels arises, whichresults in lowered uniformity of the screen.

To address this problem, there has been proposed a configuration aimingto allow the light-emission luminance of the organic EL element to bekept constant without being affected by the age deterioration of the I-Vcharacteristic of the organic EL element and changes over time in thethreshold voltage Vth and mobility μ of the drive transistor.Specifically, in this configuration, each of pixel circuits is providedwith a compensation function against change in the characteristic of theorganic EL element, and correction functions for correction againstvariation in the threshold voltage Vth of the drive transistor(hereinafter, referred to as “threshold correction”) and correctionagainst variation in the mobility μ of the drive transistor(hereinafter, referred to as “mobility correction”) (refer to e.g.Japanese Patent Laid-Open No. 2006-133542 (hereinafter referred to asPatent Document 1)).

By thus providing each pixel circuit with the compensation functionagainst change in the characteristic of the organic EL element and thecorrection functions against variation in the threshold voltage Vth andmobility μ of the drive transistor, the light-emission luminance of theorganic EL element can be kept constant without being affected by theage deterioration of the I-V characteristic of the organic EL elementand changes over time in the threshold voltage Vth and mobility μ of thedrive transistor.

The compensation function against change in the characteristic of theorganic EL element is implemented through the following series ofcircuit operation. Initially, at the timing when a video signal suppliedvia a signal line is written by a write transistor and is held in aholding capacitor connected between the gate and source of the drivetransistor, the write transistor is turned to the non-conductive stateto thereby electrically separate the gate electrode of the drivetransistor from the signal line for turning the gate electrode to thefloating state.

If the gate electrode of the drive transistor enters the floating state,in response to change in the source potential Vs of the drivetransistor, the gate potential Vg of the drive transistor also changesin linkage with (in such a manner as to follow) the change in the sourcepotential Vs, due to the connection of the holding capacitor between thegate and source of the drive transistor. This is a bootstrap operation.Due to the bootstrap operation, the gate-source voltage Vgs of the drivetransistor can be kept constant. Thus, even when the I-V characteristicof the organic EL element changes over time, the light-emissionluminance of the organic EL element can be kept constant.

In this bootstrap operation, the ratio of the rise amount ΔVg of thegate potential Vg of the drive transistor to the rise amount ΔVs of thesource potential Vs thereof (hereinafter, referred to as a bootstrapratio Gbst) is an important factor. Specifically, if this bootstrapratio Gbst is low, the gate-source voltage Vgs of the drive transistorbecomes lower than the voltage obtained at the timing when the videosignal is held in the holding capacitor.

The low bootstrap ratio Gbst is equivalent to the fact that the riseamount ΔVg of the gate potential Vg is small with respect to the riseamount ΔVs of the source potential Vs. Therefore, the gate-sourcevoltage Vgs becomes lower. This leads to failure in ensuring of thecurrent necessary as the drive current to be applied to the organic ELelement, i.e., the current corresponding to the video signal written bythe write transistor, which results in luminance lowering. Thus,luminance unevenness occurs, which causes the deterioration of the imagequality.

The bootstrap ratio Gbst depends on the capacitance of the holdingcapacitor and the capacitances of parasitic capacitors attaching to thegate of the drive transistor. The higher the capacitances of thesecapacitors are, the higher the bootstrap ratio Gbst is (details of thispoint will be described later). The capacitance of the parasiticcapacitances depends on circuit elements, such as a transistor,connected to the gate electrode of the drive transistor. If the numberof elements included in the pixel circuit is reduced and consequentlythe number of transistors connected to the gate electrode of the drivetransistor is decreased, the capacitance of the parasitic capacitorsbecomes lower correspondingly.

Therefore, increasing the capacitance of the holding capacitor iseffective to increase the bootstrap ratio Gbst. The capacitance of theholding capacitor is in proportion to the area of two metal electrodesthat form the holding capacitor and are disposed to face each other, andis in inverse proportion to the distance between these two metalelectrodes. Therefore, the capacitance of the holding capacitor can beincreased by increasing the area of two metal electrodes or decreasingthe distance between two metal electrodes. Because there is a limit tothe decreasing of the distance between two metal electrodes, theincreasing of the area of two metal electrodes, i.e., increasing of thesize of the holding capacitor, is preferentially attempted.

However, because the holding capacitor is formed under the condition ofthe limited pixel size, there is also a limit to the increasing of thesize of the holding capacitor. On the contrary, the recent trend towardminiaturization of the pixel size accompanying definition enhancementmakes it difficult to increase the bootstrap ratio Gbst through theincreasing of the size of the holding capacitor.

As another scheme, it would be available to ensure the drive currentcorresponding to the video signal by, instead of increasing thebootstrap ratio Gbst, originally designing a large current as the drivecurrent to be applied to the organic EL element via the drive transistorin anticipation of the voltage decrease corresponding to the bootstrapratio Gbst. However, this scheme involves a problem of an increase inthe power consumption.

SUMMARY OF THE INVENTION

There is a need for the present invention to provide a display that isallowed to have enhanced image quality without an increase in the powerconsumption through designing of a high bootstrap ratio under thecondition of the limited pixel size, and to provide a method formanufacturing the display, and electronic apparatus employing thedisplay.

According to an embodiment of the present invention, there is provided adisplay including pixels that are arranged in a matrix, each having anelectro-optical element, a write transistor for writing a video signal,a drive transistor for driving the electro-optical element based on thevideo signal written by the write transistor, and a holding capacitorconnected between the gate and source of the drive transistor. In thisdisplay, the holding capacitor is formed by using a first electrode, asecond electrode disposed to face one surface of the first electrode forforming a first capacitor, and a third electrode disposed to face theother surface of the first electrode for forming a second capacitor, andthe first capacitor and the second capacitor are connected in parallelto each other electrically.

According to the display having this configuration and the electronicapparatus including this display, by forming the first and secondcapacitors by using the first, second, and third electrodes andconnecting these first and second capacitors in parallel to each otherelectrically, the capacitance of the holding capacitor can be increasedwithout increasing the size of the holding capacitor compared with aholding capacitor formed by using two electrodes, if the distances amongthe first, second, and third electrodes are equalized to each other.Thus, a high bootstrap ratio can be designed under the condition of thelimited pixel size.

Because the bootstrap ratio can be set high, when the source potentialof the drive transistor rises up, the rise amount of the gate potentialis substantially equal to the rise amount of the source potential inbootstrap operation, and thus the gate-source voltage of the drivetransistor is not reduced. This allows ensuring of the current necessaryas the drive current to be applied to the electro-optical elementwithout originally designing a large current as the drive current inanticipation of the voltage decrease corresponding to the bootstrapratio.

According to the embodiment of the present invention, a high bootstrapratio can be designed under the condition of the limited pixel size, andthus the current necessary as the drive current to be applied to theelectro-optical element can be ensured without originally designing alarge current as the drive current in anticipation of the voltagedecrease corresponding to the bootstrap ratio. This can enhance theimage quality without increasing the power consumption.

Furthermore, if the same capacitance as that of a capacitor formed byusing two electrodes is designed without attempting to increase thecapacitance of the holding capacitor, the area of the electrodes of theholding capacitor can be decreased. Thus, the ratio of the area of theelectrodes of the holding capacitor to the total area of the pixel isreduced. For example, this makes it possible to suppress the occurrenceof short-circuit due to a metal dust or the like that enters the pixelin the manufacturing process and hence a fault in the pixel, which canenhance the yield.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system configuration diagram showing the schematicconfiguration of an organic EL display according to one embodiment ofthe present invention;

FIG. 2 is a circuit diagram showing a specific configuration example ofa pixel (pixel circuit);

FIG. 3 is a sectional view showing one example of the sectionalstructure of the pixel;

FIG. 4 is a timing chart for explaining the operation of the organic ELdisplay according to the embodiment;

FIGS. 5A to 5D are explanatory diagrams of circuit operation in theorganic EL display according to the embodiment;

FIGS. 6A to 6D are explanatory diagrams of the circuit operation in theorganic EL display according to the embodiment;

FIG. 7 is a characteristic diagram for explaining a problem due tovariation in threshold voltage Vth of a drive transistor;

FIG. 8 is a characteristic diagram for explaining a problem due tovariation in mobility μ of the drive transistor;

FIGS. 9A to 9C are characteristic diagrams for explaining therelationship between signal voltage Vsig of a video signal anddrain-source current Ids of the drive transistor in association with thepresence/absence of threshold correction and mobility correction;

FIG. 10 is an equivalent circuit diagram of the pixel;

FIG. 11 is a circuit diagram showing the connection relationship amongthe respective circuit elements in a pixel, including the electricconnection of a holding capacitor;

FIG. 12 is a planar pattern diagram showing the structure ofinterconnect layers in the pixel according to the embodiment;

FIG. 13 is a sectional view showing the structure of a holding capacitoraccording to one working example of the present invention;

FIG. 14 is a planar pattern diagram showing the structure ofinterconnect layers in a pixel according to related art;

FIG. 15 is a step diagram showing a method for manufacturing the holdingcapacitor according to one working example of the present invention;

FIG. 16 is a sectional view showing the structure of a holding capacitoraccording to another working example of the present invention;

FIG. 17 is a perspective view showing a television to which theembodiment is applied;

FIGS. 18A and 18B are perspective views showing a digital camera towhich the embodiment is applied: FIG. 18A is a front-side view and FIG.18B is a rear-side view;

FIG. 19 is a perspective view showing a notebook personal computer towhich the embodiment is applied;

FIG. 20 is a perspective view showing a video camera to which theembodiment is applied; and

FIGS. 21A to 21G are perspective views showing a cellular phone to whichthe embodiment is applied: FIGS. 21A and 21B are a front view and sideview, respectively, of the opened state, and FIGS. 21C to 21G are afront view, left-side view, right-side view, top view, and bottom view,respectively, of the closed state.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described in detail belowwith reference to the accompanying drawings.

FIG. 1 is a system configuration diagram showing the schematicconfiguration of an active-matrix display according to one embodiment ofthe present invention. As one example, the following description willdeal with an active-matrix organic. EL display that employs, as alight-emitting element of each pixel (pixel circuit), an organic ELelement, which is a current-driven electro-optical element whoselight-emission luminance varies depending on the current flowing throughthe element.

As shown in FIG. 1, an organic EL display 10 according to the presentembodiment includes a pixel array part 30 in which pixels (PXLC) 20 aretwo-dimensionally arranged on rows and columns (in a matrix).Furthermore, the organic EL display 10 includes a driving part that isdisposed in the periphery of the pixel array part 30 and drives therespective pixels 20. The driving part includes a write scan circuit 40,a power supply scan circuit 50, and a horizontal drive circuit 60 as thedriving part for example.

In the pixel array part 30, corresponding to the pixel arrangement of mrows and n columns, scan lines 31-1 to 31-m and power supply lines 32-1to 32-m are provided for the respective pixel rows, and signal lines33-1 to 33-n are provided for the respective pixel columns.

The pixel array part 30 is typically formed on a transparent insulatingsubstrate such as a glass substrate, and has a flat-type panelstructure. Each of the pixels 20 in the pixel array part 30 can beformed by using an amorphous silicon thin film transistor (TFT) or alow-temperature poly-silicon TFT. In the case of using thelow-temperature poly-silicon TFT, the scan circuit 40, the power supplyscan circuit 50, and the horizontal drive circuit 60 can also be mountedon a display panel (substrate) 70 on which the pixel array part 30 isformed.

The write scan circuit 40 includes a shift register for sequentiallyshifting (transferring) a start pulse sp in synchronization with a clockpulse ck. For writing of the video signal to the pixels 20 in the pixelarray part 30, the write scan circuit 40 sequentially supplies scansignals WS1 to WSm to the scan lines 31-1 to 31-m, for sequentialscanning of the pixels 20 on a row-by-row basis (line-sequentialscanning).

The power supply scan circuit 50 includes a shift register forsequentially shifting the start pulse sp in synchronization with theclock pulse ck. In synchronization with the line-sequential scanning bythe write scan circuit 40, the power supply scan circuit 50 supplies thepower supply lines 32-1 to 32-m with power supply line potentials DS1 toDSm that are each switched between a first potential Vccp and a secondpotential Vini lower than the first potential Vccp, to thereby controlthe conductive state (ON)/non-conductive state (OFF) of a drivetransistor 22 (see FIG. 2) to be described later.

The horizontal drive circuit 60 properly selects either one of a signalvoltage Vsig and an offset voltage Vofs of the video signal that issupplied from a signal supply source (not shown) and corresponds toluminance information, and writes the video signal to the pixels 20 inthe pixel array part 30 via the signal lines 33-1 to 33-n simultaneouslye.g. in units of rows. That is, the horizontal drive circuit 60 employsa drive form for line-sequential writing in which the signal voltageVsig of the video signal is simultaneously written in units of rows(lines).

The offset voltage Vofs is the voltage serving as the reference(equivalent to e.g. the black level) of the signal voltage (hereinafter,it will be often referred to simply as “signal voltage”) Vsig of thevideo signal. The second potential Vini is set to a potentialsufficiently lower than the offset voltage Vofs; it is set to e.g. apotential that satisfies the relationship Vofs−Vth>Vini, in which Vthdenotes the threshold voltage of the drive transistor 22.

(Pixel Circuit)

FIG. 2 is a circuit diagram showing a specific configuration example ofthe pixel (pixel circuit) 20. As shown in FIG. 2, the pixel 20 includes,as a light-emitting element, an organic EL element 21, which is acurrent-driven electro-optical element whose light-emission luminancevaries depending on the current flowing through the element. In additionto the organic EL element 21, the pixel 20 includes a drive transistor22, a write transistor 23, and a holding capacitor 24.

In this combination, N-channel TFTs are used as the drive transistor 22and the write transistor 23. This combination of the conductivity typesof the drive transistor 22 and the write transistor 23 is merely oneexample, and the combination is not limited thereto.

The cathode electrode of the organic EL element 21 is connected to acommon power supply line 34 that is provided in common to all the pixels20. The source electrode of the drive transistor 22 is connected to theanode electrode of the organic EL element 21, and the drain electrode ofthe drive transistor 22 is connected to the power supply line 32 (32-1to 32-m).

The gate electrode of the write transistor 23 is connected to the scanline 31 (31-1 to 31-m). One electrode (source electrode/drain electrode)of the write transistor 23 is connected to the signal line 33 (33-1 to33-n), and the other electrode (drain electrode/source electrode)thereof is connected to the gate electrode of the drive transistor 22.

One end of the holding capacitor 24 is connected to the gate electrodeof the drive transistor 22, and the other end thereof is connected tothe source electrode of the drive transistor 22 (the anode electrode ofthe organic EL element 21). It is also possible to employ aconfiguration in which an auxiliary capacitor is connected in parallelto this holding capacitor 24 to thereby compensate for any insufficiencyof the capacitance of the holding capacitor 24.

In the pixel 20 having such a configuration, the write transistor 23enters the conductive state in response to the scan signal WS applied tothe gate electrode thereof from the write scan circuit 40 via the scanline 31. Thereby, the write transistor 23 samples the signal voltageVsig or the offset voltage Vofs of the video signal that is suppliedfrom the horizontal drive circuit 60 via the signal line 33 and dependson luminance information, and writes the sampled voltage in the pixel20. This written input signal voltage vsig or offset voltage Vofs isheld in the holding capacitor 24.

When the potential DS of the power supply line 32 (32-1 to 32-m) is atthe first potential Vccp, the drive transistor 22 is supplied withcurrent from the power supply line 32 and supplies the organic ELelement 21 with drive current having the current value dependent uponthe voltage value of the input signal voltage Vsig held in the holdingcapacitor 24, to thereby drive the organic EL element 21 by the current.

(Pixel Structure)

FIG. 3 is a sectional view showing one example of the sectionalstructure of the pixel 20. As shown in FIG. 3, the pixel 20 has astructure in which an insulating film 202, an insulating planarizationfilm 203 and a window insulating film 204 are formed sequentially over aglass substrate 201 on which the pixel circuit composed of the drivetransistor 22, the write transistor 23, and so on is formed, and theorganic EL element 21 is provided in a recess 204A of the windowinsulating film 204.

The organic EL element 21 is composed of an anode electrode 205 that isformed at the bottom of the recess 204A of the window insulating film204 and is composed of a metal or the like, an organic layer (electrontransport layer, light-emitting layer, hole transport layer/holeinjection layer) 206 formed on the anode electrode 205, and a cathodeelectrode 207 that is formed on the organic layer 206 in common to allthe pixels and is formed of a transparent conductive film or the like.

In this organic EL element 21, the organic layer 206 is formed bysequentially depositing over the anode electrode 205 a hole transportlayer/hole injection layer 2061, a light-emitting layer 2062, anelectron transport layer 2063, and an electron injection layer (notshown). Under driving by current by the drive transistor 22 of FIG. 2,current flows through the organic layer 206 from the drive transistor 22via the anode electrode 205. This causes the recombination betweenelectrons and holes in the light-emitting layer 2062 in the organiclayer 206, and light is emitted in response to this recombination.

As shown in FIG. 3, after the organic EL element 21 is formed for eachpixel over the glass substrate 201 on which the pixel circuit is formedwith the intermediary of the insulating film 202, an insulatingplanarization film 203 and the window insulating film 204, a sealingsubstrate 209 is bonded by an adhesive 210 with the intermediary of apassivation film 208 so that the organic EL element 21 is sealed by thesealing substrate 209. This completes the display panel 70.

(Threshold Correction Function)

In the period during which the horizontal drive circuit 60 supplies theoffset voltage Vofs to the signal line 33 (33-1 to 33-m) after the writetransistor 23 enters the conductive state, the power supply scan circuit50 switches the potential DS of the power supply line 33 from the secondpotential Vini to the first potential Vccp. Due to the switching of thepotential DS of the power supply line 32, the voltage equivalent to thethreshold voltage Vth of the drive transistor 22 is held in the holdingcapacitor 24.

The holding of the voltage equivalent to the threshold voltage Vth ofthe drive transistor 22 in the holding capacitor 24 is for the followingreason.

Due to variation in the manufacturing process for the drive transistor22 and change in the drive transistor 22 over time, there is variationin the transistor characteristics of the drive transistor 22, such asthe threshold voltage Vth and the mobility μ, from each pixel. Due tothe variation in transistor characteristics, even when the same gatepotential is applied to the drive transistor 22 among the pixels, thedrain-source current (drive current) Ids varies from pixel to pixel, andthis current variation will appear as variation in the light-emissionluminance of the organic EL element 21. In order to cancel (correct) theinfluence, of the variation in the threshold voltage Vth from pixel topixel, the voltage equivalent to the threshold voltage Vth is held inthe holding capacitor 24.

The correction of the threshold voltage Vth of the drive transistor 22is carried out as follows. Specifically, the threshold voltage Vth isheld in the holding capacitor 24 in advance. Due to this operation, inthe driving of the drive transistor 22 by the signal voltage Vsig of thevideo signal, the threshold voltage Vth of the drive transistor 22 andthe voltage that is held in the holding capacitor 24 and is equivalentto the threshold voltage Vth cancel each other. In other words, thecorrection of the threshold voltage Vth is carried out.

This corresponds to the threshold correction function. Due to thisthreshold correction function, even when there is variation and changeover time in the threshold voltage Vth from pixel to pixel, thelight-emission luminance of the organic EL element 21 can be keptconstant without being affected by the variation and the change overtime. The principle of the threshold correction will be described indetail later.

(Mobility Correction Function)

The pixel 20 shown in FIG. 2 is provided with the mobility correctionfunction in addition to the above-described threshold correctionfunction. Specifically, during the period when the horizontal drivecircuit 60 supplies the signal voltage Vsig of the video signal to thesignal line 33 (33-1 to 33-n) and the write transistor 23 is in theconductive state in response to the scan signal WS (WS1 to WSm) outputfrom the write scan circuit 40, i.e., during the mobility correctionperiod, mobility correction of cancelling the dependence of thedrain-source current Ids of the drive transistor 22 on the mobility μ iscarried out in the holding of the input signal voltage Vsig in theholding capacitor 24. The specific principle and operation of thismobility correction will be described later.

(Bootstrap Function)

The pixel 20 shown in FIG. 2 is also provided with a bootstrap function.Specifically, at the timing when the signal voltage Vsig of the videosignal has been held in the holding capacitor 24, the write scan circuit40 stops the supply of the scan signal WS (WS1 to WSm) to the scan line31 (31-1 to 31-m) to thereby switch the write transistor 23 to thenon-conductive state for electrically isolating the gate electrode ofthe drive transistor 22 from the signal line 33 (33-1 to 33-n). Thus,the gate electrode of the drive transistor 22 enters the floating state.

If the gate electrode of the drive transistor 22 enters the floatingstate, in response to change in the source potential Vs of the drivetransistor 22, the gate potential Vg of the drive transistor 22 alsochanges in linkage with (in such a manner as to follow) the change inthe source potential Vs, due to the connection of the holding capacitor24 between the gate and source of the drive transistor 22 (bootstrapoperation). Due to this bootstrap operation, even when the I-Vcharacteristic of the organic EL element 21 changes over time, thelight-emission luminance of the organic EL element 21 can be keptconstant.

Specifically, even when the I-V characteristic of the organic EL element21 has changed over time and correspondingly the source potential Vs ofthe drive transistor 22 has changed, the gate-source voltage Vgs of thedrive transistor 22 is kept constant due to the bootstrap operation.Thus, the current flowing through the organic EL element 21 does notchange, and hence the light-emission luminance of the organic EL element21 is also kept constant. Consequently, even when the I-V characteristicof the organic EL element 21 changes over time, image displaying withoutluminance deterioration accompanying the change over time is permitted.

(Circuit Operation of Organic EL Display)

Subsequently, the circuit operation of the organic EL display 10according to the present embodiment will be described below based on thetiming chart of FIG. 4 with reference to the operation explanatorydiagrams of FIGS. 5 and 6. In the operation explanatory diagrams ofFIGS. 5 and 6, the write transistor 23 is represented by a switch symbolfor simplification of the drawings. Furthermore, because the organic ELelement 21 has a parasitic capacitor Cel, this parasitic capacitor Celis also shown in these diagrams.

In the timing chart of FIG. 4, changes in the following potentials atthe 1-H (H denotes horizontal scanning period) are shown along the sametime axis: the potential (scan signal) WS of the scan line 31 (31-1 to31-m), the potential DS of the power supply line 32 (32-1 to 32-m), andthe gate potential Vg and source potential Vs of the drive transistor22.

<Light-Emission Period>

According to the timing chart of FIG. 4, the organic EL element 21 is inthe light-emission state before time t1 (light-emission period). In thislight-emission period, the potential DS of the power supply line 32 isat the higher potential Vccp (first potential), and the write transistor23 is in the non-conductive state. The drive transistor 22 is sodesigned as to operate in the saturation region in this period.Therefore, as shown in FIG. 5A, the drive current (drain-source current)Ids dependent upon the gate-source voltage Vgs of the drive transistor22 is supplied to the organic EL element 21 from the power supply line32 via the drive transistor 22. This causes the organic EL element 21 toemit light with the luminance dependent upon the current value of thedrive current Ids.

<Threshold Correction Preparation Period>

At the time t1, a new field of the line-sequential scanning starts, andas shown in FIG. 5B, the potential DS of the power supply line 32 isswitched from the higher potential Vccp to the potential Vini (secondpotential) sufficiently lower than the offset voltage Vofs of the signalline 33. Under the definition that the threshold voltage of the organicEL element 21 is Vel and the potential of the common power supply line34 is Vcath, if the lower potential Vini is set to satisfy therelationship Vini<Vel+Vcath, the organic EL element 21 enters thereverse-bias state and thus the light emission thereof stops because thesource potential Vs of the drive transistor 22 becomes almost equal tothe lower potential Vini.

Subsequently, the potential WS of the scan line 31 is switched from alower potential side to a higher potential side at time t2 so that thewrite transistor 23 enters the conductive state, as shown in FIG. 5C. Atthis time, the offset voltage Vofs is supplied from the horizontal drivecircuit 60 to the signal line 33. Thus, the gate potential Vg of thedrive transistor 22 becomes equal to the offset voltage Vofs. The sourcepotential Vs of the drive transistor 22 is at the potential Vinisufficiently lower than the offset voltage Vofs.

At this time, the gate-source voltage Vgs of the drive transistor 22 isVofs−Vini. At this time, unless this voltage Vofs−Vini is higher thanthe threshold voltage Vth of the drive transistor 22, theabove-described threshold correction operation may not be carried out.Thus, the potentials should be so designed as to satisfy therelationship Vofs−Vini>Vth. This operation of fixing (settling) the gatepotential Vg of the drive transistor 22 at the offset voltage Vofs andfixing the source potential Vs at the lower potential Vini forinitialization in this manner corresponds to the operation of thethreshold correction preparation.

<Threshold Correction Period>

Subsequently, as shown in FIG. 5D, upon switching of the potential DS ofthe power supply line 32 from the lower potential Vini to the higherpotential Vccp at the time t3, the source potential Vs of the drivetransistor 22 starts to rise. In due course, the source potential Vs ofthe drive transistor 22 reaches the potential Vofs−Vth. At this time,the gate-source voltage Vgs of the drive transistor 22 is equal to thethreshold voltage Vth of the drive transistor 22 so that the voltageequivalent to this threshold voltage Vth is written to the holdingcapacitor 24.

In the present specification, the period for writing the voltageequivalent to the threshold voltage Vth in the holding capacitor 24 isreferred to as the threshold correction period, for convenience. Thepotential Vcath of the common power supply line 34 is so designed thatthe organic EL element 21 is kept at the cut-off state in this thresholdcorrection period in order that current does not flow toward the organicEL element 21 but flows toward the holding capacitor 24 exclusively inthe threshold correction period.

Subsequently, the potential WS of the scan line 31 is shifted to thelower potential side at time 4 so that the write transistor 23 entersthe non-conductive state as shown in FIG. 6A. At this time, the gateelectrode of the drive transistor 22 becomes the floating state.However, because the gate-source voltage Vgs is equal to the thresholdvoltage Vth of the drive transistor 22, the drive transistor 22 is inthe cut-off state. Therefore, the drain-source current Ids does not flowto the drive transistor 22.

<Writing Period/Mobility Correction Period>

Subsequently, as shown in FIG. 6B, the potential of the signal line 33is switched from the offset voltage Vofs to the signal voltage Vsig ofthe video signal at time t5. And then the potential WS of the scan line31 is shifted to the higher potential side at time t6. Due to thisoperation, as shown in FIG. 6C, the write transistor 23 enters theconductive state to sample the signal voltage Vsig of the video signaland write the sampled voltage in the pixel 20.

Due to the writing of the signal voltage Vsig by the write transistor23, the gate potential Vg of the drive transistor 22 becomes the signalvoltage Vsig. In driving of the drive transistor 22 by the signalvoltage Vsig of the video signal, the threshold voltage Vth of the drivetransistor 22 and the voltage that is held in the holding capacitor 24and is equivalent to the threshold voltage vth cancel each other so thatthe threshold correction is carried out. The principle of the thresholdcorrection will be described later.

At this time, the organic EL element 21 is initially in the cut-offstate (high-impedance state). Therefore, the current (drain-sourcecurrent Ids) supplied from the power supply line 32 through the drivetransistor 22, depending on the signal voltage Vsig of the video signal,flows into the parasitic capacitor Cel of the organic EL element 21, andthus the charging of the parasitic capacitor Cel is started.

Due to the charging of this parasitic capacitor Cel, the sourcepotential Vs of the drive transistor 22 rises up along with the elapseof time. At this time, variation in the threshold voltage Vth of thedrive transistor 22 has been already corrected, and the drain-sourcecurrent Ids of the drive transistor 22 depends on the mobility μ of thedrive transistor 22.

In due course, the source potential Vs of the drive transistor 22 risesup to the potential Vofs−Vth+ΔV. At this time, the gate-source voltageVgs of the drive transistor 22 is equal to Vsig−Vofs+Vth−ΔV.Specifically, the rise of the source potential Vs by the amount ΔVfunctions to subtract the potential ΔV from the voltage (Vsig−Vofs+Vth)held in the holding capacitor 24. In other words, this potential risefunctions to discharge the electric charges in the holding capacitor 24,which is equivalent to negative feedback. Consequently, the rise amountΔV of the source potential Vs is equivalent to the feedback amount ofthe negative feedback.

By thus carrying out the negative feedback of the drain-source currentIds flowing through the drive transistor 22 to the gate input of thedrive transistor 22, i.e., to the gate-source voltage Vgs, mobilitycorrection for cancelling the dependence of the drain-source current Idsof the drive transistor 22 on the mobility μ, i.e., for correctingvariation in the mobility μ from pixel to pixel, is carried out.

More specifically, when the signal voltage Vsig of the video signal ishigher, the drain-source current Ids is larger and thus the absolutevalue of the feedback amount (correction amount) ΔV of the negativefeedback is also larger. Therefore, the mobility correction dependentupon the light-emission luminance level is carried out. Furthermore,when the signal voltage Vsig of the video signal is constant, becausehigher mobility μ of the drive transistor 22 provides a larger absolutevalue of the feedback amount ΔV of the negative feedback, variation inthe mobility μ from pixel to pixel can be eliminated. The principle ofthe mobility correction will be described later.

<Light-Emission Period>

Subsequently, the potential WS of the scan line 31 is shifted to thelower potential side at time t7 so that the write transistor 23 entersthe non-conductive state, as shown in FIG. 6D. This isolates the gateelectrode of the drive transistor 22 from the signal line 33.Simultaneously, the drain-source current Ids starts flowing through theorganic EL element 21, which raises the anode potential of the organicEL element 21 depending on the drain-source current Ids.

The rise of the anode potential of the organic EL element 21 isequivalent to the rise of the source potential Vs of the drivetransistor 22. In linkage with the rise of the source potential Vs ofthe drive transistor 22, the gate potential Vg of the drive transistor22 also rises up due to the bootstrap operation of the holding capacitor24. The rise amount of the gate potential Vg is equal to that of thesource potential Vs. Therefore, during the light-emission period, thegate-source voltage Vgs of the drive transistor 22 is kept constant atVsig−Vofs+Vth−ΔV. At the time t8, the potential of the signal line 33 isswitched from the signal voltage Vsig of the video signal to the offsetvoltage Vofs.

(Principle of Threshold Correction)

The principle of the threshold correction for the drive transistor 22will be described below. The drive transistor 22 operates as a constantcurrent source because it is so designed as to operate in the saturationregion. Thus, the constant drain-source current (drive current) Idsrepresented by Equation (1) is supplied from the drive transistor 22 tothe organic EL element 21.

Ids=(½)·μ(W/L)Cox(Vgs−Vth)²  (1)

In Equation (1), W denotes the channel width of the drive transistor 22,L denotes the channel length thereof, and Cox denotes the gatecapacitance per unit area.

FIG. 7 shows the characteristic of the drain-source current Ids vs. thegate-source voltage Vgs of the drive transistor 22. As shown in thischaracteristic diagram, unless correction against variation in thethreshold voltage Vth of the drive transistor 22 is carried out, thesame voltage Vgs provides the different drain source currents Ids.Specifically, when the threshold voltage Vth is Vth1, the drain-sourcecurrent Ids corresponding to the gate-source voltage Vgs is Ids1. On theother hand, when the threshold voltage Vth is Vth2 (Vth2>Vth1), thedrain-source current Ids corresponding to the same gate-source voltageVgs is Ids2 (Ids2<Ids1). That is, if there is variation in the thresholdvoltage Vth of the drive transistor 22, the drain-source current Idsvaries even when the gate-source voltage Vgs is constant.

In contrast, in the pixel (pixel circuit) 20 having the above-describedconfiguration, the gate-source voltage Vgs of the drive transistor 22 inthe light-emission state is Vsig−Vofs+Vth−ΔV, as described above.Therefore, the drain-source current Ids is represented by Equation (2),which is obtained by substituting this voltage for Vgs in Equation (1).

Ids=(½)·μ(W/L)Cox(Vsig−Vofs−ΔV)²  (2)

That is, the term of the threshold voltage Vth of the drive transistor22 is cancelled, and thus the drain-source current Ids supplied from thedrive transistor 22 to the organic EL element 21 does not depend on thethreshold voltage Vth of the drive transistor 22. As a result, even whenthe threshold voltage Vth of the drive transistor 22 varies from pixelto pixel due to variation in the manufacturing process for the drivetransistor 22 and changes in the drive transistor 22 over time, thedrain-source current Ids will not vary and hence the light-emissionluminance of the organic EL element 21 will also not vary.

(Principle of Mobility Correction)

The principle of the mobility correction for the drive transistor 22will be described below. FIG. 8 shows characteristic curves forcomparison between a pixel A in which the mobility μ of the drivetransistor 22 is relatively high and a pixel B in which the mobility μof the drive transistor 22 is relatively low. If the drive transistor 22is formed of a poly-silicon thin film transistor or the like, it isinevitable that the mobility μ varies from pixel to pixel like themobility difference between the pixel A and the pixel B.

If the input signal voltage Vsig of the same level is written to boththe pixels A and B, for example, in the state in which the mobility μ isdifferent between the pixel A and the pixel B, no correction of themobility μ yields large a difference between the drain-source currentIds1′ flowing through the pixel A in which the mobility μ is high andthe drain-source current Ids2′ flowing through the pixel B in which themobility μ is low. The occurrence of such a large difference in thedrain-source current Ids between pixels attributed to variation in themobility μ will lower the uniformity of the screen.

As is apparent from the above-described transistor characteristicequation (1), higher mobility μ provides larger drain-source currentIds. Therefore, the higher the mobility μ is, the larger the feedbackamount ΔV of the negative feedback is. As shown in FIG. 8, the feedbackamount ΔV1 of the pixel A having high mobility μ is larger than thefeedback amount ΔV2 of the pixel B having low mobility μ. Thus, bycarrying out negative feedback of the drain-source current Ids of thedrive transistor 22 to the input signal voltage Vsig by the mobilitycorrection operation, variation in the mobility μ can be suppressedbecause higher mobility μ provides a higher degree of the negativefeedback.

Specifically, due to the correction by the feedback amount ΔV1 for thepixel A having high mobility μ, the drain-source current Ids therein isgreatly decreased from Ids1′ to Ids1. On the other hand, because thefeedback amount ΔV2 of the pixel B having low mobility μ is small, thedecrease in the drain-source current Ids therein is not so large: fromIds2′ to Ids2. As a result, the drain-source current Ids1 of the pixel Aand the drain-source current Ids2 of the pixel B become almost equal toeach other so that the variation in the mobility μ is corrected.

Summarizing the above, when pixel A and pixel B have different mobilityμ, the feedback amount ΔV1 of the pixel A having high mobility μ islarger than the feedback amount ΔV2 of the pixel B having low mobilityμ. That is, in a pixel with higher mobility μ, the feedback amount ΔV islarger and thus the decrease amount of the drain-source current Ids islarger. Consequently, through the negative feedback of the drain-sourcecurrent Ids of the drive transistor 22 to the input signal voltage Vsig,the current values of the drain-source currents Ids in pixels havingdifferent mobility μ are equalized. As a result, the variation in themobility μ can be corrected.

With use of FIGS. 9A to 9C, a description will be made below about therelationships between the signal potential (sampling potential) Vsig ofthe video signal and the drain-source current Ids of the drivetransistor 22 in the pixel (pixel circuit) 20 shown in FIG. 2, inassociation with the presence/absence of the threshold correction andthe mobility correction.

FIG. 9A shows the case in which neither the threshold correction nor themobility correction is carried out. FIG. 9B shows the case in which themobility correction is not carried out, but the threshold correction iscarried out. FIG. 9C shows the case in which both the thresholdcorrection and the mobility correction are carried out. As shown in FIG.9A, if neither the threshold correction nor the mobility correction iscarried out, a large difference in the drain-source current Ids arisesbetween the pixels A and B attributed to variation in the thresholdvoltage Vth and the mobility μ between the pixels A and B.

In contrast, if only the threshold correction is carried out, variationin the drain-source current Ids can be decreased to some extent by thethreshold correction, as shown in FIG. 9B. However, difference in thedrain-source current Ids between the pixels A and B still remainsattributed to variation in the mobility p between the pixels A and B.

If both the threshold correction and the mobility correction are carriedout, as shown in FIG. 9C, the difference in the drain-source current Idsbetween the pixels A and B attributed to variation in the thresholdvoltage Vth and the mobility μ between the pixels A and B can besubstantially eliminated. Thus, variation in the luminance of theorganic EL element 21 does not arise at any grayscale, which can providea display having a favorable image quality.

Advantageous Effects of Present Embodiment (Problem in BootstrapOperation)

A problem in the bootstrap operation will be described below with use ofFIG. 10, which shows an equivalent circuit of the pixel 20.

In FIG. 10, Cs denotes the capacitance of the holding capacitor 24, Cgsdenotes the capacitance of the parasitic capacitor between the gate andsource of the drive transistor 22, Cgd denotes the capacitance of theparasitic capacitor between the gate and drain of the drive transistor22, and Cd denotes the capacitance of the parasitic capacitor betweenthe gate and drain/source of the write transistor 23. If the rise amountof the source potential Vs of the drive transistor 22 is represented asΔVs and the rise amount of the gate potential Vg thereof is representedas ΔVg, the ratio of the rise amount ΔVg of the gate potential Vg of thedrive transistor to the rise amount ΔVs of the source potential Vsthereof, i.e., the bootstrap ratio Gbst, is represented by Equation (3).

Gbst=ΔVg/ΔVs={(Cs+Cgs)/(Cs+Cgs+Cgd+Cd)}  (3)

In the bootstrap operation, if the bootstrap ratio Gbst is low, the riseamount ΔVg of the gate potential Vg is small with respect to the riseamount ΔVs of the source potential Vs, and thus the gate-source voltageVgs is reduced and becomes lower than the voltage corresponding to thesignal voltage Vsig held in the holding capacitor 24 through writing bythe write transistor 23.

This leads to failure in the ensuring of the current necessary as thedrive current Ids, to be applied to the organic EL element 21, i.e., thecurrent corresponding to the signal voltage Vsig of the video signalwritten by the write transistor 23, which results in luminance lowering.Thus, luminance unevenness occurs, which causes the deterioration of theimage quality.

As is apparent from Equation (3), the bootstrap ratio Gbst is determinedby the capacitance Cs of the holding capacitor 24 and the parasiticcapacitances Cgs, Cgd, and Cd attaching to the gate of the drivetransistor 22. The higher these capacitances Cs, Cgs, Cgd, and Cd are,the higher the bootstrap ratio Gbst is.

Feature of Present Embodiment

A feature of the present embodiment is that the capacitance Cs of theholding capacitor 24 is increased without increasing the area of theelectrodes of the holding capacitor 24, i.e., the size of the holdingcapacitor 24, for the above-described reason (the reason described inDescription of The Related Art), to thereby design a high bootstrapratio Gbst under the condition of the limited pixel size for enhancedimage quality.

Specifically, in the present embodiment for the holding capacitor 24, afirst electrode composed of a metal is interposed between second andthird electrodes each composed of a metal in such a way that the secondand third electrodes face the respective surfaces of the firstelectrode. Thereby, a first capacitor 24-1 is formed between the firstand second electrodes, and a second capacitor 24-2 is formed between thefirst and third electrodes. These first and second capacitors 24-1 and24-2 are connected in parallel to each other electrically.

FIG. 11 is a circuit diagram showing the connection relationship amongthe respective circuit elements in the pixel 20, including the electricconnection of the holding capacitor 24 (first and second capacitors 24-1and 24-2). In FIG. 11, the same part as that in FIG. 2 is given the samenumeral.

Referring to FIG. 11, the holding capacitor 24 is composed of the firstcapacitor 24-1 formed between a first electrode 24A and a secondelectrode 24B disposed to face one surface of the electrode 24A, and thesecond capacitor 24-2 formed between the first electrode 24A and a thirdelectrode 24C disposed to face the other surface of the electrode 24A.These first and second capacitors 24-1 and 24-2 are connected inparallel to each other electrically.

FIG. 12 is a planar pattern diagram showing the structure ofinterconnect layers in the pixel 20. Referring to FIGS. 11 and 12, thesecond electrode 24B and an interconnect 241 thereof are formed of afirst metal layer, which is the same as the layer of the gate electrodeof the drive transistor 22. The first electrode 24A and an interconnect242 thereof are formed of a second metal layer, which is the same as thelayer of the source electrode of the drive transistor 22 and the otherelectrode (drain electrode/source electrode) of the write transistor 23.The third electrode 24C is formed of a third metal layer, which is thesame as the layer of the anode electrode of the organic EL element 21.

The interconnect 242 of the first electrode 24A is connected to the gateelectrode of the drive transistor 22 via a contact 25 for electricallyconnecting the first metal layer to the second metal layer. Theinterconnect 241 of the second electrode 24B is connected to the sourceelectrode of the drive transistor 22 via a contact 26 for electricallyconnecting the first metal layer to the second metal layer. The sourceelectrode of the drive transistor 22 in the second metal layer iselectrically connected to the anode electrode of the organic EL element21 in the third metal layer via a contact 27.

As is apparent from this electric connection relationship, the holdingcapacitor 24 is formed by using the first electrode 24A formed of thesecond metal layer at the same potential as that of the gate electrodeof the drive transistor 22, the second electrode 24B formed of the firstmetal layer at the same potential as that of the source electrode of thedrive transistor 22, and the third electrode 24C formed of the thirdmetal layer at the same potential as that of the source electrode of thedrive transistor 22.

The second electrode 24B of the holding capacitor 24 and the gateelectrode of the drive transistor 22 are at different potentials,although both are formed of the first metal layer. Furthermore, thefirst electrode 24A of the holding capacitor 24 and the source electrodeof the drive transistor 22 are at different potentials, although bothare formed of the second metal layer.

Advantageous Effects of Present Embodiment

By thus forming the first and second capacitors 24-1 and 24-2 by usingthe first, second, and third electrodes 24A, 24B, and 24C and connectingthese first and second capacitors 24-1 and 24-2 in parallel to eachother electrically, the capacitance Cs of the holding capacitor 24 canbe increased (can be doubled, in principle) compared with the holdingcapacitor 24 formed by using two electrodes, without increasing the sizeof the holding capacitor 24, if the distances among three electrodes24A, 24B, and 24C are equalized to each other. This allows designing ofa high bootstrap ratio Gbst under the condition of the limited pixelsize.

Because the bootstrap ratio Gbst can be set high, and ideally can be setto one, when the source potential Vs of the drive transistor 22 risesup, the rise amount ΔVg of the gate potential Vg is substantially equalto the rise amount ΔVs of the source potential Vs in bootstrapoperation, and thus the gate-source voltage Vgs is not reduced. Thisallows ensuring of the current necessary as the drive current Ids to beapplied to the organic EL element 21.

This feature can eliminate luminance lowering and accompanying luminanceunevenness that occur unless the drive current Ids corresponding to thesignal voltage Vsig of the video signal written by the write transistor23 can be ensured, without originally designing a large current as thedrive current to be applied to the organic EL element 21 in anticipationof the voltage decrease corresponding to the bootstrap ratio Gbst. Thus,the image quality can be enhanced without an increase in the powerconsumption.

In the case of the organic EL display 10 employing the pixelconfiguration shown in FIG. 2, the transistor connected to the drivetransistor 22 is only the write transistor 23. Therefore, the number oftransistors connected to the drive transistor 22 is smaller comparedwith the organic EL display described in Patent document 1 (JapanesePatent Laid-Open No. 2006-133542) and so on. Thus, the parasiticcapacitance attaching to the gate electrode of the drive transistor 22is correspondingly lower, and hence the bootstrap ratio Gbst tends to below. Consequently, the technique for increasing the capacitance Cs ofthe holding capacitor 24 without increasing the size of the holdingcapacitor 24 is particularly effective for the organic EL display 10employing the pixel configuration shown in FIG. 2.

Other Advantageous Effects of Present Embodiment

Even if there is no need to increase the capacitance Cs of the holdingcapacitor 24, by forming the first and second capacitors 24-1 and 24-2by using three electrodes 24A, 24B, and 24C and connecting these firstand second capacitors 24-1 and 24-2 in parallel to each otherelectrically, the area of the electrodes of the holding capacitor 24 canbe reduced compared with the holding capacitor 24 formed by using twoelectrodes, if the distances among three electrodes 24A, 24B, and 24Care equalized to each other. In principle, the area can be reduced tothe half.

The feature that the area of the electrodes of the holding capacitor 24can be reduced is equivalent to the feature that the size of the holdingcapacitor 24 can be decreased, and therefore can contribute tominiaturization of the pixel 20. Furthermore, if the area of theelectrodes of the holding capacitor 24 is reduced while the pixel sizeis kept as it is without the miniaturization of the pixel 20, the ratioof the area of the electrodes of the holding capacitor 24 to the totalarea of the pixel 20 is decreased. Thus, even when a metal dust or thelike enters the pixel 20 in the manufacturing process, for example, itis possible to suppress the occurrence of the short-circuit between theelectrode of the holding capacitor 24 and another metal interconnect dueto the metal dust or the like and hence a fault in the pixel 20, whichcan enhance the yield.

Working Example

A specific working example will be described below. FIG. 13 is asectional view along line A-A′ in FIG. 12, showing the structure of theholding capacitor 24 according to one working example of the presentinvention. In FIG. 13, the same part as that in FIG. 3 is given the samenumeral.

As shown in FIG. 13, the holding capacitor 24 is formed by using thesecond electrode 24B, the first electrode 24A, and the anode electrode205 of the organic EL element 21. The second electrode 24B is formed ofa pattern of the first metal layer on a transparent insulating substratesuch as a glass substrate 201. The first electrode 24A is formed of apattern of the second metal layer disposed over the second electrode 24Bwith the intermediary of an insulating film 202. The anode electrode 205is formed of a pattern of the third metal layer disposed over the firstelectrode 24A with the intermediary of an insulating protective film211.

Of the anode electrode 205 of the organic EL element 21, the partialportion (area) opposed to the first electrode 24A serves as the thirdelectrode 24C. The first capacitor 24-1 is formed between the secondelectrode 24B and the first electrode 24A, and the second capacitor 24-2is formed between the first electrode 24A and the third electrode 24C.As described above, these first and second capacitors 24-1 and 24-2 areconnected in parallel to each other electrically so as to form theholding capacitor 24.

A feature of the present working example is that an insulatingplanarization film 203, which is originally provided between theinsulating protective film 211 and the third metal layer, is partiallyremoved from the area through which the first electrode 24A faces thethird electrode 24C (area 243 surrounded by the dashed-dotted line inFIG. 12).

For this partial removal of the insulating planarization film 203, thepatterning mask for forming the pattern of the insulating planarizationfilm 203 is so fabricated as to have a pattern for providing an aperturecorresponding to the area of the holding capacitor 24. This allows theinsulating planarization film 203 to be removed from the area throughwhich the first electrode 24A faces the third electrode 24C in theforming of the pattern of the insulating planarization film 203, withoutan increase in the number of steps.

By thus removing the insulating planarization film 203 from the areathrough which the first electrode 24A faces the third electrode 24C, thedistance d1 between both the electrodes 24A and 24C can be decreased tothe value equivalent to the film thickness of the insulating protectivefilm 211. As is well known, the capacitance is in proportion to the areaof two electrodes and is inversely proportional to the distance betweentwo electrodes.

Therefore, due to the decreasing of the distance d1 between the firstand third electrodes 24A and 24C, the capacitance of the secondcapacitor 24-2 formed between these electrodes 24A and 24C can beincreased to almost the same capacitance as that of the first capacitor24-1 formed between the first and second electrodes 24A and 24B, unlikethe case in which the insulating planarization film 203, which generallyhas a thickness several times that of the insulating protective film211, exists between the electrodes 24A and 24C.

The present working example will be compared with related art shown inFIG. 14, in which the holding capacitor 24 is formed by using the firstelectrode 24A of the second metal layer and the partial portion of thefirst metal layer opposed to the first electrode 24A (second electrode24B). This comparison is based on an assumption that the area of thefirst electrode 24A shown in FIG. 12 is equal to that of the firstelectrode 24A shown in FIG. 14.

According to this comparison, the present working example is allowed toensure the capacitance about twice that in the related art shown in FIG.14 as the capacitance Cs of the holding capacitor 24, because the secondcapacitor 24-2 formed between the first and third electrodes 24A and 24Ccan be increased to almost the same capacitance as that of the firstcapacitor 24-1 formed between the first and second electrodes 24A and24B by removing the insulating planarization film 203 from the areathrough which the first electrode 24A faces the third electrode 24C.

(Method for Manufacturing Holding Capacitor)

A method for manufacturing the holding capacitor 24 according to thepresent working example will be described below with use of the stepdiagram shown in FIG. 15. In FIG. 15, the same part as that in FIG. 13is given the same numeral.

Initially, the second electrode 24B is formed as a pattern of the firstmetal layer on a transparent insulating substrate such as the glasssubstrate 201 (Step 1), and then the insulating film 202 is formed tocover the second electrode 24B (Step 2). Subsequently, the firstelectrode 24A is formed as a pattern of the second metal layer (Step 3),and then the insulating protective film 211 is formed to cover the firstelectrode 24A (Step 4).

Subsequently, the insulating planarization film 203 is formed as apattern by fabricating a patterning mask by photoresist. The patterningmask is so fabricated that the mask is formed also on the area opposedto the first electrode 24A so that the insulating planarization film 203is formed as the pattern (Step 5). By thus forming the insulatingplanarization film 203 as the pattern, the partial portion of theinsulating planarization film 203 opposed to the first electrode 24A canbe removed simultaneously with the step of forming the insulatingplanarization film 203.

Subsequently, the anode electrode 205 of the organic EL element 21 isformed as a pattern of the third metal layer (Step 6). At this time,because the partial portion of the insulating planarization film 203opposed to the first electrode 24A is removed, the anode electrode 205is formed on the insulating protective film 211 in this removed part.The partial portion of the anode electrode 205 under which theinsulating planarization film 203 is removed serves as the thirdelectrode 24C.

As described above, in the step of forming the insulating planarizationfilm 203 (Step 5), the insulating planarization film 203 is removed fromthe area between the first electrode 24A and the third electrode 24C.Thus, the distance d1 between the first electrode 24A and the thirdelectrode 24C can be set small which can increase the capacitance of thesecond capacitor 24-2 formed between the electrodes 24A and 24C withoutan increase in the number of steps.

Other Working Examples

In the above-described working example, the insulating planarizationfilm 203 is removed from the area through which the first electrode 24Afaces the third electrode 24C, as a preferred mode of the embodiment.However, as shown in FIG. 16, the insulating planarization film 203 doesnot necessarily need to be removed from this area. Even in the case ofFIG. 16, the second capacitor 24-2 having the capacitance correspondingto the distance d2, which is equivalent to the sum of the film thicknessof the insulating planarization film 203 and the film thickness of theinsulating protective film 211, can be formed between the firstelectrode 24A and the third electrode 24C.

Furthermore, by making an electric connection in such a way that thesecond capacitor 24-2 is connected in parallel to the first capacitor24-1, as described above, the capacitance Cs of the holding capacitor 24arising from the parallel connection between the first capacitor 24-1and the second capacitor 24-2 can be increased corresponding to theexistence of the second capacitor 24-2.

Modification Examples

In the above-described embodiment, as an example, its feature is appliedto the organic EL display 10 having the pixel circuit 20 with thecircuit configuration that includes the drive transistor 22 for drivingthe organic EL element 21, the write transistor 23 for sampling thesignal voltage Vsig of the video signal and writing it in the pixel, andthe holding capacitor 24 that is connected between the gate and sourceof the drive transistor 22 and holds the signal voltage Vsig written bythe write transistor 23. However, the present invention is not limitedto this application example.

The embodiment can be similarly applied also to organic EL displayshaving any of the following pixel circuits: a pixel circuit having aswitching transistor that is connected between the drive transistor 22and a power supply line and operates to selectively supply drive currentfrom the power supply line to the drive transistor 22; and a pixelcircuit further having a switching transistor that enters the conductivestate timely to thereby operate to detect the threshold voltage Vth ofthe drive transistor 22 and hold the detected threshold voltage vth inthe holding capacitor 24 before driving of the organic EL element 21 bycurrent.

Moreover, in the above-described embodiment, the drive scheme is appliedto an organic EL display that employs an organic EL element as anelectro-optical element in the pixel circuit 20, as an example. However,the embodiment is not limited to this application example but can beapplied to overall displays that employ a current-driven electro-opticalelement (light-emitting element) whose light-emission luminance variesdepending on the value of the current flowing through the element.

Application Examples

The display according to the above-described embodiment can be appliedto various kinds of electronic apparatus shown in FIGS. 17 to 21 as oneexample. Specifically, it can be used as a display in electronicapparatus in any field that displays, as an image or video, a videosignal input thereto or a video signal produced therein, such as adigital camera, notebook personal computer, portable terminal apparatustypified by a cellular phone, and video camera.

Using the display according to the embodiment as a display in electronicapparatus in any field offers an advantage that images having highquality can be displayed at low power consumption in various kinds ofelectronic apparatus, because the display according to the embodiment isallowed to have enhanced image quality without an increase in the powerconsumption through designing of a high bootstrap ratio under thecondition of the limited pixel size, as is apparent from the descriptionof the above embodiment.

The display according to the embodiment encompasses also a module-shapedisplay with a sealed structure. Examples of such a display include adisplay module formed by attaching a counterpart composed of transparentglass or the like to the pixel array part 30. This transparentcounterpart may be provided with a color filter, protective film,light-shielding film, and so on. The display module may be provided witha circuit part, flexible printed circuit (FPC), and so on forinput/output of signals and so forth to/from the pixel array partfrom/to the external.

Examples of electronic apparatus to which the embodiment is applied willbe described below.

FIG. 17 is a perspective view showing a television to which theembodiment is applied. This television includes a video display screen101 composed of a front panel 102, a filter glass 103, and so on, and isfabricated by using the display according to the embodiment as the videodisplay screen 101.

FIGS. 18A and 18B are perspective views showing a digital camera towhich the embodiment is applied: FIG. 18A is a front-side view and FIG.18B is a rear-side view. This digital camera includes a light emitter111 for flash, a display part 112, a menu switch 113, a shutter button114, and so on, and is fabricated by using the display according to theembodiment as the display part 112.

FIG. 19 is a perspective view showing a notebook personal computer towhich the embodiment is applied. This notebook personal computerincludes in its main body 121 a keyboard 122 operated in inputting ofcharacters and so forth, a display part 123 for displaying images, andso on. The notebook personal computer is fabricated by using the displayaccording to the embodiment as the display part 123.

FIG. 20 is a perspective view showing a video camera to which theembodiment is applied. This video camera includes a main body 131, alens 132 that is disposed on the front side of the camera and used tocapture a subject image, a start/stop switch 133 for imaging operation,a display part 134, and so on. The video camera is fabricated by usingthe display according to the embodiment as the display part 134.

FIGS. 21A to 21G are perspective views showing a cellular phone as anexample of portable terminal apparatus to which the embodiment isapplied: FIGS. 21A and 21B are a front view and side view, respectively,of the opened state, and FIGS. 21C to 21G are a front view, left-sideview, right-side view, top view, and bottom view, respectively, of theclosed state. This cellular phone includes an upper casing 141, a lowercasing 142, a connection (hinge) 143, a display 144, a sub-display 145,a picture light 146, a camera 147, and so on. The cellular phone isfabricated by using the display according to the embodiment as thedisplay 144 and the sub-display 145.

It should be understood by those skilled in the art that variousmodifications, combinations, sub-combinations and alterations may occurdepending on design requirements and other factor in so far as they arewithin the scope of the appended claims or the equivalents thereof.

1. A display comprising: a pixel array part configured to include pixelsthat are arranged in a matrix and each have an electro-optical element,a write transistor for writing a video signal, a drive transistor fordriving the electro-optical element based on the video signal written bythe write transistor, and a holding capacitor connected between gate andsource of the drive transistor, wherein the holding capacitor includes afirst electrode, a second electrode disposed to face one surface of thefirst electrode for forming a first capacitor, and a third electrodedisposed to face the other surface of the first electrode for forming asecond capacitor, and the first capacitor and the second capacitor areconnected in parallel to each other electrically.
 2. The displayaccording to claim 1, wherein the pixel array part has a first metallayer that is used to form a gate electrode of the drive transistor, asecond metal layer that is used to form a source electrode of the drivetransistor and is provided above the first metal layer with intermediaryof an insulating film, and a third metal layer that is used to form ananode electrode of the electro-optical element and is provided above thesecond metal layer with intermediary of an insulating protective filmand an insulating planarization film, the first electrode is formed ofthe second metal layer, the second electrode is formed of the firstmetal layer, and the third electrode is formed of the third metal layer.3. The display according to claim 2, wherein of the insulatingprotective film and the insulating planarization film that areinterposed between the second metal layer and the third metal layer, theinsulating planarization film is absent between the first electrode andthe third electrode.
 4. The display according to claim 2, wherein thefirst electrode is connected to the gate electrode of the drivetransistor via a first contact for electrically connecting the firstmetal layer to the second metal layer, and the second electrode isconnected to the source electrode of the drive transistor via a secondcontact for electrically connecting the first metal layer to the secondmetal layer.
 5. A method for manufacturing a display including pixelsthat are arranged in a matrix and each have an electro-optical element,a write transistor for writing a video signal, a drive transistor fordriving the electro-optical element based on the video signal written bythe write transistor, and a holding capacitor connected between gate andsource of the drive transistor, the holding capacitor including a firstelectrode, a second electrode disposed to face one surface of the firstelectrode for forming a first capacitor, and a third electrode disposedto face the other surface of the first electrode for forming a secondcapacitor, the first capacitor and the second capacitor being connectedin parallel to each other electrically, the method comprising the stepsof: forming a first metal layer for forming a gate electrode of thedrive transistor and the second electrode on a substrate; forming asecond metal layer for forming a source electrode of the drivetransistor and the first electrode above the first metal layer withintermediary of an insulating film; forming an insulating planarizationfilm above the second metal layer with intermediary of an insulatingprotective film; and forming a third metal layer for forming an anodeelectrode of the electro-optical element and the third electrode on theinsulating planarization film, wherein in the forming the insulatingplanarization film, the insulating planarization film is removed from anarea between the first electrode and the third electrode.
 6. Electronicapparatus including a display, the display comprising: pixels configuredto be arranged in a matrix and each have an electro-optical element, awrite transistor for writing a video signal, a drive transistor fordriving the electro-optical element based on the video signal written bythe write transistor, and a holding capacitor connected between gate andsource of the drive transistor, wherein the holding capacitor includes afirst electrode, a second electrode disposed to face one surface of thefirst electrode for forming a first capacitor, and a third electrodedisposed to face the other surface of the first electrode for forming asecond capacitor, and the first capacitor and the second capacitor areconnected in parallel to each other electrically.